While it is possible to make passable color prints using the full visible spectrum for printing illumination, higher color saturation and contrast is secured if the printing light is limited to certain narrow bands of red, green and blue regions of the spectrum corresponding to the three regions where the sensitized material has its peak sensitivity and the dyes forming the negative image have their maximum density.
Eastman Kodak, in their manual "Kodak Filters for Scientific and Technical Uses" (First Edition, page 35) designate filters suitable for printing color negatives. They name their Wratten filters numbered 70, 99, and 98 to isolate narrow bands within the red, green and blue light components respectively. These are absorption type filters which perform the same in any of light stream, convergent, divergent, collimated, or any combination of these.
The red filter, number 70, is quite satisfactory, cutting off the unwanted energy sharply, while offering little obstruction to the desired energy band. This is not true for the other two filters. They also remove the unwanted wavelengths, but at the expense of absorbing a major share of the useful light. The blue filter, number 98, passes only 40% of the incident energy at its highest transmission point of 430 nanometers wavelength. The green filter, number 99, passes only 19.9% of the incident green light at 550 nanometers wavelength, its highest transmission point. Such losses cannot be tolerated in competitive commercial photo printing. Instead, the industry turned to dichroic filters and "white light subtractive printing".
A dichroic filter operates by transmitting light of one portion of the spectrum, while reflecting the rest of the spectrum. Employed as a filter the reflected portion is discarded. The fact that such filters divide the spectrum into two complimentary parts makes them extremely useful as beam splitters or as synthesizers for combining different colored light streams.
These filters are highly effective if their unique limitation can be accommodated. The spectral content of the transmitted and reflected components will vary from its designed value if the light stream is not collimated and incident to the filter at its designed angle (usually normal).
When precise control of transmission is needed a collimator has been required, but these cannot provide nearly enough illumination to compete with light sources in present use.
Consequently, the industry has almost universally adopted the full spectrum as emitted by high wattage incandescent lamps for a printing illumination.
It is the major object of this invention to provide a source of narrow band printing illumination, free of filter crossover, which combined with the optical system of my earlier patent can serve in high production printers.
To describe the term "narrow band" as used herein refer to FIG. 11, a graph of sensitivity vb wavelength. It will be seen that each emulsion has much higher sensitivity to a specific band of wavelength, but no emulsion is insensitive to other colors. There is a region of overlap. Narrow band printing uses illumination which is devoid of the wavelengths which would produce exposure in the overlap sections. To accomplish this, filters must be designed to transmit wavelengths narrower than the usual stock red, green, and blue dichroic filters, moreover the light used with these filters must be collimated within a certain tolerance.
Obviously exposure by white light exposes these overlap regions along with all the rest of the spectrum. Nor do commercial additive lamphouses do better by merging red, green and blue light beams. Not only are the individual dichroic filters designed to pass the full spectral band of these colors, these filters are used without collimated light.
This invention removes the wavelengths corresponding to regions of overlap. So exposures are absolutely confined to the proper emulsion.